Pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are required for follicle development, oocyte maturation and steroidogenesis in the ovary. However, locally produced growth factors, cytokines and other proteins and peptides modulate the actions of LH and FSH on ovarian cells. We have identified a novel protein in the human ovarian follicular fluid, which was named steroidogenesis-inducing protein (SIP) due to its major stimulatory effects on steroid production in testicular, ovarian and adrenal cells. Later studies revealed that SIP is also a potent growth factor and stimulates DNA synthesis in rat granulosa cells, immature rat Leydig cells and human ovarian epithelial cancer cells. The effects of SIP on target cells are mediated via a tyrosine-kinase coupled signaling pathway. We have recently purified SIP from human follicular fluid to homogeneity and analysis of its N-terminal amino acid sequence demonstrated that SIP is a novel protein that belongs to immunoglobulin superfamily of proteins. We hypothesize that SIP is secreted by ovarian granulosa cells in response to gonadotropins and, in turn, mediates the effects of tropic hormones on proliferation, differentiation and steroidogenic function of granulosa and luteal cells. In addition, an unregulated secretion of SIP in the human ovaries may play a role in development and/or maintenance of ovarian epithelial cancers. The studies proposed in the current application have been designed to enhance our current knowledge on SIP and will be focused on the following specific aims: a) to further characterize SIP protein after obtaining additional amino acid sequence and cloning of human SIP gene, development of recombinant SIP protein and reagents for analysis of SIP in biological materials, and b) to further characterize SIP produced by granulosa cells in the rat and human ovary. We will also study the regulation of SIP expression by tropic hormones (FSH, LH) and steroid hormones in the rat ovary and will determine the precise cellular source of SIP in the ovary. The results of these studies should provide valuable information on the biochemical structure and regulation of SIP and will provide valuable tools for future studies on the role of SIP in the development and function of ovarian follicle and corpus luteum during female reproductive cycle.